As I almost never comment upon current events, these blog entries have a timeless triviality. Sample the various years and see what interests you.

Sunday, March 15, 2015

Castlewood Canyon State Park

Yesterday was a perfect day for hiking.In the afternoon I ventured south to the
Castlewood Canyon State Park.Here is
a glamour shot of the canyon from the Geological Society of America.

Here is my own photograph of caprock on the east-side canyon wall.

The park's geology is complex.Here is a brief description from the Castlewood
Canyon State Park Geology pamphlet:

Millions of years ago, a tropical rainforest covered what is now Castlewood Canyon State
Park.How do we know?Because plant and animal fossils from those
tropical forests have been found in the oldest visible rocks in the park,
called Dawson Arkose.

Although the rocks above the Dawson Arkose cannot speak,
they tell the story of a tremendous volcanic eruption that occurred precisely
36.7 million years ago.The eruption,
which happened about 90 miles away near present-day Salida, filled the air with
a glowing cloud of 2,000 degree molten rock, ash and poisonous gases.It reached the area of the park in just a few
minutes.The liquid rock and superheated
ash welded into a thick layer of solid rock as they hit the ground cooled, and
were buried.This rock has several names:
ignimbrite (Latin for “fiery cloud”, Wall Mountain Tuff (named for the mountain
northeast of Salida where it was first discovered), and rhyolite. You can find
pieces of this once liquid rock laying all over the park.Look for rocks with sharp angles and edges,
tiny air holes, and shiny specs. It can be pink, purple, gray or brown in
color.

The “icing” on the park’s rock layer cake of Dawson
Arkose and rhyolite, and its most distinguishing geologic feature, is Castle
Rock Conglomerate.These 34
million-year-old rocks, washed down from the eroding Rocky Mountains, form the
park’s canyon walls and caprock. Conglomerate rocks are easy to identify –
they’re like cookie dough with bits of chocolate chips sticking out. The
“dough” is sedimentary rock and the chips are pebbles and boulders that are
smoothed and rounded in ancient rivers and cemented into the rock by the
water’s high concentration of silicates – nature’s concrete.

I saw a layer of purple rhyolite that looked
something like this.

Here is a picture I took of some Castle Rock
Conglomerate. Water-smoothed stones, ranging in size from baseballs to cantaloupes, were embedded in the surrounding rock layer.